This invention relates to a fuel supply control method for internal combustion engines under high load conditions, and more particularly to a fuel supply control method of this kind which is adapted to enrich a mixture being supplied to the engine so as to avoid an excessive rise in the bed temperature of a catalyst device provided in the engine when the engine is operating in high load regions.
In internal combustion engines in general, a mixture being supplied to the engine is enriched when the engine is operating under high load conditions such as at quick acceleration, so as to achieve required high engine output, thereby improving the driveability of the engine.
On the other hand, a catalyst device provided in the exhaust system of an internal combustion engine is generally adapted to show maximum conversion efficiency of exhaust gas ingredients and exhibit the best exhaust gas purifying function when the air-fuel ratio of the mixture assumes a value equal to a stoichiometric mixture ratio or a value in the vicinity thereof. However, in a high load region of the engine, as described later, the catalyst device has an increased reaction rate when the mixture assumes an air-fuel ratio equal to the stoichiometric ratio or a value in the vicinity thereof. Depending upon the operating conditions of the engine, the temperature of the catalyst bed can rise to an excessive degree, even resulting in burning of the catalyst bed. Such excessive rise of the bed temperature can be restrained by enriching the mixture as mentioned above. However, enriching of the mixture will cause a degradation in the exhaust gas purifying function of the catalyst device.
In view of the above circumstances, it is necessary to discriminate proper mixture-enriching high load regions of an internal combustion engine from other operating regions of the engine so as to achieve prevention of excessive rise of the catalyst bed temperature as well as to improve the output performance and emission characteristics of the engine, at the same time.
Conventionally, setting of such mixture-enriching high load regions of an internal combustion engine has been made on the basis of the throttle valve opening.
The catalyst bed temperature increases with an increase in the weight flow rate of intake air G.sub.AIR drawn into the engine, and the weight flow rate of air is in turn variable as a function of the rotational speed of the engine and the absolute pressure within the intake pipe of the engine. Therefore, if in a high speed region of the engine where the weight flow rate of air is relatively large the air-fuel ratio of the mixture is set to a value equal to a stoichiometic mixture ratio or a value in the vicinity thereof, the catalyst bed temperature can rise excessively, providing a larger possibility of burning of the catalyst bed than in a low speed region of the engine.
To solve this problem, it has been proposed, e.g. by Japanese Provisional Utility Model Publication No. 53-22928 to set a predetermined throttle valve opening value for determining a mixture-enriching high load region of an internal combustion engine, to a predetermined value smaller in a high speed region of the engine than in a low speed region of the engine. However, since this proposed method depends solely upon the throttle valve opening to set the mixture-enriching high load region of the engine throughout the whole engine rotational speed range, enrichment of the mixture cannot take place in a certain region in the above high engine speed region where the throttle valve opening is smaller than the above predetermined value and the intake pipe absolute pressure is higher than a certain value, thereby providing a possibility of excessive rise of the catalyst bed temperature.
In view of the aforementioned fact that the catalyst bed temperature varies in response to the weight flow rate of air drawn into the engine being a function of the engine rotational speed and the intake pipe absolute pressure, proper setting of the mixture-enriching high load region of the engine can be made for better prevention of excessive rise of the catalyst bed temperature, if such setting is made on the basis of the intake pipe absolute pressure as well as the engine rotational speed.
Further, in high altitudes the weight flow rate of air drawn into the engine is smaller than in lowlands so that the possibility of excessive rise of the catalyst bed temperature is reduced when the engine is operating in high altitudes, so long as the engine is operating under the same operating conditions as in lowlands. Therefore, according to the above proposed method depending upon the throttle valve opening alone to set the mixture-enriching high load region, if it is desired to obtain the same results during engine operation in high altitudes as in lowlands, it will be required to correct the predetermined throttle valve opening value for determining the mixture-enriching high load region to a larger value than that applied during engine operation in lowlands. This makes complicate in structure a fuel supply control system to which the method is applied.
Moreover, provided that the throttle valve opening is maintained at a constant value, the intake pipe absolute pressure lowers as the engine rotational speed increases. Therefore, if setting of the mixture-enriching high load region of the engine is made on the basis of the engine rotational speed and the intake pipe absolute pressure alone, there can exist a region where the mixture is not enriched even when the operator steps on the accelerator pedal to accelerate the engine. Thus, in such region required accelerating performance cannot be obtained, but also the catalyst bed temperature can rise excessively.